1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of seismic data acquisition. Specifically, the invention is a method for 3D seismic acquisition, using near real time illumination monitoring.
2. Description of the Related Art
Seismic surveying is a method for determining structures of rock formations below the earth's surface. Seismic surveys are performed by generating seismic signals at source locations and receiving the resulting seismic signals at receiver locations. The seismic signal emanates from the source, and spreads outwardly in a substantially spherical pattern. When the signal reaches interfaces in the subsurface between strata having different acoustic velocities or different acoustic impedances, a portion of the energy will be reflected from the interface, and a portion of the energy will be transmitted through the interface. For the reflected energy, the ray path follows the rule that the angle of incidence is equal to the angle of reflection. The ray path of the energy traveling through the interface will be altered according to Snell's Law.
If the earth's surface and all reflecting interfaces in the subsurface are substantially flat and parallel to the earth's surface, then for seismic signals generated at a given source location and detected at a given receiver location, it can be assumed that the reflection locations will be directly below the mid-point between the source location and the receiver location. Accordingly, if seismic signals are generated at an evenly spaced pattern of source locations, such as a rectangular grid of locations having uniform spacing of the source locations in both the in-line and cross-line directions and if the receiver locations are also evenly spaced, then the locations at which the resulting seismic signals are reflected from a given subsurface reflecting interface will also be evenly spaced, and the reflecting interface will be uniformly illuminated. As used here, the term “illumination” refers to the quantity of signals reflected from a designated area of a subsurface reflecting interface and detected by the receivers.
Frequently, subsurface structures that are of interest are neither parallel to the earth's surface nor flat. Because of the irregularities in the structure of this body, the seismic ray paths emanating from the surface seismic sources that travel to this body and are then reflected to the earth's surface will reach the earth's surface in a very irregular pattern, so that the signals received by the receivers positioned in a standard regular grid pattern as described above represent an illumination of the body which has substantial variation from one part of the structure to another. For areas of low illumination, the fold may be inadequate to map such locations satisfactorily. As used here, the term “fold” refers to the number of received seismic data traces representative of the areas of illumination.
Roy, J. et al., U.S. Pat. No. 6,560,565 B2, “Satellite-Based Seismic Mobile Information and Control System”, issued May 6, 2003, discloses a real time data gathering, quality control, and information distribution system, comprising field resources, satellite resources, and office resources that are located at a different site from the field. The seismic system uses positional data determined by GPS (Global Positioning System) integrated with IMU (Inertial Measuring Unit) to determine in near real time whether the determined set of coordinates of a location in the field is within specification relative to quality control parameters and with respect to a set of pre-plot coordinates. A mobile unit determines its coordinates and sends them along with quality control parameters via satellite communications and the Internet to the information and control center in the office resources, where a knowledge base containing facts and expert rules is used to determine if the actual coordinates are sufficiently close to the pre-plot coordinates given the associated quality control parameters. If a mismatch has occurred, a solution is initially and automatically formulated using the knowledge base, and then reviewed and approved by human experts at the control center. A final decision is then transmitted to the mobile unit in the field resources, via satellite communications and the Internet, before the field crew leaves the site. All of the information and actions are shared with the appropriate personnel within the group carrying out the work, as well as with the client and their quality control subcontractors, over the Internet. Accordingly, personnel in the field can reposition the equipment while they are still at the site of the equipment. However, Roy et al. do not correct for the illumination coverage of the target horizons.
Traditional methods for determining if a 3D (three-dimensional) seismic survey has achieved adequate coverage for data quality purposes has been CMP (common midpoint) multiplicity or fold analysis. This analysis calculates the locations of the midpoints and offsets between every source and receiver combination throughout the survey area. These computations are typically segmented into offset ranges and then displayed graphically. If the number of source and receiver combinations that fall into any grid location or bin in the survey falls below certain criteria, infill data acquisition is indicated. This methodology calculates the coverage of the 3D acquisition from the perspective of obtaining uniform surface distributions of reflection points and offsets over the survey area. This method of fold analysis has proven useful, but may not provide an adequate picture of the subsurface illumination in a situation of complicated geology.
It has long been known that there is insufficient resemblance between the coverage of 3D acquisition based on surface calculations, as described in the prior art, when compared to the seismic illumination at a prospective target horizon. A useful tool that can be used in conjunction with the surface coverage calculations is a calculation of the ray trace illumination coverage at the target horizon (or several target horizons). Ray trace illumination modeling has been used for the design of 3D acquisition parameters and for the analysis of 3D seismic illumination after the acquisition phase is complete. Tools and methods exist that can model the predicted illumination at a target horizon based on idealized acquisition parameters, or to analyze the coverage achieved based on navigation and location data recorded during the acquisition phase of a survey. Ray tracing through an earth model can produce a more accurate estimation of subsurface fold coverage and provide a basis for comparison of candidate geometries.
Campbell, S. B. et al., “Comparative ray-based illumination analysis”, Soc. of Exp. Geophys., Int'l. Exp. and 72nd Ann. Mtg., Salt Lake City, Utah, Oct. 6-11, 2002, Exp. Abstracts, pp. 41-44 discloses a method for targeting illumination with a model-based, ray-tracing scheme. This method is described further in U.S. patent application Ser. No. 10/155,158, Campbell, Steven B., “Targeted Geophysical Survey”, filed May 24, 2002 by a co-inventor of the present application and assigned to the assignee of the present application. Ray tracing is applied to a target area containing hypothetical source and receiver arrays in dense patterns. Emergent points are collected for all contributing sources and plotted as if they were source/receiver midpoints in a fold diagram. The diagram shows areas of high emergent fold for the receivers, revealing the most efficient placement of receivers. Ray tracing may be applied again with only the high emergent receiver locations and the hypothetical dense array of sources. Departure points are now binned as source/receiver midpoints and areas of high departure fold reveal the most efficient placement of sources. However, Campbell et al. do not discuss how to correct for the illumination coverage of the target horizons in real or near real time.
Ray trace illumination modeling from actual navigation data is a better approximation of the degree to which a subsurface target horizon has been illuminated than conventional surface-based common midpoint coverage analysis. However, to be of any use in the planning and execution of infill acquisition, the results of the ray trace illumination modeling needs to be available in real time. The ray trace illumination results must be available almost immediately at the completion of acquisition of the data, or better still, while the acquisition of the seismic data is still in progress. Recent advances in the speed at which data can be transferred and improvements in computational abilities have made this possible.
Therefore, a need exists for a method to provide an improved data set that can be used in the analysis of infill decisions during the course of a 3D seismic acquisition project. Thus, a need exists for a method for monitoring the illumination of 3D seismic acquisition in near real time. A need exists for a methodology for acquiring and then analyzing subsurface illumination modeling results, determining and applying the preferred location of infill acquisition, and predicting and evaluating the results of the infill acquisition, all in near real time.